The present invention relates to the use of monolithic catalysts in multi-tubular catalytic reactors. More particularly, the invention relates to catalyst designs and methods for inserting, securing, and maintaining monolithic catalysts in the tubes of such reactors, or in shell-and-tube heat exchangers, for use in the chemical processing and/or energy conversion industries.
Tubular catalytic reactors wherein a reactant stream is passed through a tube containing a bed of catalyst pellets, rings, spheres, or the like are presently used for the industrial production of chemicals in a variety of processes. These include processes involving highly exothermic or endothermic reactions wherein the management of the heat of reaction is required for process control. Examples of highly exothermic reactions include the selective catalytic oxidation of organic compounds e.g., the oxidation of benzene or n-butane to maleic anhydride, o-xylene to phthalic anhydride, methanol to formaldehyde, ethylene to ethylene oxide, and Fischer-Tropsch synthesis. Highly endothermic reactions include the steam reforming of hydrocarbons to syngas (CO and H2). For all of these reactions, effective heat management can significantly affect key process parameters including catalyst efficiency, reaction selectivity, adequate catalyst life, and even reactor safety.
Tubular reactors are relatively efficient but can be difficult to control. In the case of exothermic reactions for example, hot spots can occur which can adversely affect reactor performance. Due to effects such as process stream flow channeling and the fact that the effective thermal conductivity of the reaction system (catalyst pellets plus gaseous reactants) is quite low, localized heating that increases exothermic reaction rates can produce thermal runaways. Uncontrolled, these can eventually lead to catalyst sintering or melting, damage to metal reactor envelopes, and even to reactor explosions. Approaches to deal with these concerns have included processing strategies such as staging the catalysts or diluting the reactants, the latter through means such as reaction moderators, product recycling or the use of inert diluents, but such strategies invariably reduce process efficiencies.
Multi-tubular reactors offer a more efficient method for securing reaction zone temperature control. These reactors typically contain a large number of tubes, typically of the order of centimeters in diameter, loaded with packed pellet catalysts. The range of reaction zone temperature control can be increased by reducing tubular reactor diameter and/or increasing the volume, flow, or heat capacity of the various heat exchange fluids such as gases, water, thermal oil, and molten salts that have been used. Further, flow-channeling effects of the kind leading to thermal runaways in pelletized catalyst beds can be minimized or eliminated through the substitution of structured packings, e.g., monolithic or honeycomb catalysts, for the pelletized catalysts in the tubes.
The use of thermally conductive, structured metal honeycomb catalysts to improve thermal uniformity in tubular reactors has been proposed in publications by E. Tronconi and G. Groppi, including “Design Of Novel Monolith Catalyst Supports For Gas/Solid Reactions With Heat Exchange”, Chem. Eng. Sci. 55 (2000), 2161-2171. Modeling work by these authors and others suggest that appropriately designed conductive monoliths could offer significant reductions in catalyst temperature gradients in tubular reactors via heat conduction through the interconnected walls of the monoliths. However, problems relating to overall reactor temperature control remain.
One difficulty with any of the multi-tubular reactor designs so far considered is that the heat generated or required by the reaction must still be supplied or removed through the tube walls via the heat exchange medium present in the space in between the reactor tubes. Thus a major contributor to the problem of catalyst superheating in exothermic reactions is the physical limitation on internal heat transfer performance that can be achieved in these reactors. This physical limitation, expressed commonly as the heat transfer coefficient or the effective radial thermal conductivity of the reactor, is frequently still too low in comparison with the amount of heat evolved inside the reactor tube to enable the level of reactor temperature control needed to realize theoretical reactor efficiencies.
Monolithic catalysts to be used in multi-tubular reactors themselves present additional practical difficulties, specifically problems relating to the efficient loading and fitting of the catalysts into commercial reactor tubes. Neither the reactor tubes nor the catalysts themselves are ideal in shape, and therefore gaps between the catalysts and the tube walls inevitably remain. Such gaps further increase the heat transfer resistance between the catalyst and heat transfer fluid within the reactor. Thus improved methods for packaging monolithic catalysts in reactor tubes are needed to minimize the resistance to heat transfer arising from the series of interfaces and materials disposed between the catalyst and the heat transfer fluid within the reactor.